So, not sure if its just the way i am using it or if its a bug in the sketcher. But, if you were to create 2 rings vertically aligned on the XY Plane (so 2 circles at the top - offset 4mm and 2 circles on the bottom - offset 4 mm), that shape won't be able to pad.
But, in V0.16 - it works fine.
If you repeat the steps below in 0.16 and pad the part - it works fine. Then if you open the file in 0.17 - it looks fine.
But you cannot change the padding / remove and repad.

Steps To Reproduce

1. Launch Freecad 0.17
2. Start a new file
3. Activate Part Design workbench
4. Create a new part / body
5. Create a new sketch - xy plane
6. Draw a circle centered on the origin - constrain to 11.2 mm
7. Draw a circle centered on the origin - constrain to 15.2 mm
8. Draw a circle centered on Y-Axis, X+32mm - constrain to 8mm
9. Draw a circle centered on Y-Axis, X+32mm - constrain to 12 mm
10. At this point - The part is fully constrained.
11. Close the sketcher
12. Select Pad
One of 2 thing happens:
a. Sketch disappears - nothing is padded
b. Only one of the rings is padded